Shellac is a well-known commercial resin which originates as a secretion of an insect, Laccifer lacca or Tachardia lacca, found in Eastern countries, such as India, Pakistan and Sri Lanka. Principal components of shellac include aleuritic acid, shellolic acid and jalaric acid. Under some conditions, shellac can polymerize. However, shellac is not generally considered to be a polymer.
Shellac has been used as a coating on some foods and medications in order to improve their appearance. Examples of foods on which shellac has been applied include apples and confections. Forms of medications on which shellac has been employed as a coating include pills and tablets.
Although shellac coatings are enteric and non-toxic, shellac exhibits physical properties which make formation of such coatings problematic. For example, shellac is not water-soluble and has a melting point in the range of between about 75.degree. and 80.degree. C. Therefore, in order to minimize damage to surfaces to which it is applied, shellac typically must be dissolved in a medium before it is applied to a surface. The solvent can then be evaporated to leave a shellac coating.
A common method of forming shellac coatings on perishable foods and medications includes dissolving shellac in an alcohol solvent. Examples of typical solvents include denatured ethanol. However, federal regulatory requirements limit the use of ethanol. Further, explosion-proof equipment is typically required when handling alcohol-based solutions. Also, organic solvent-based solutions, such as alcohol-based solutions, typically have a residual amount of solvent in the coating. In addition, shellac components of alcohol-based coatings can polymerize over time, thereby diminishing their ability to dissolve in the digestive track and consequently reducing their value as enteric coatings.
Another method of forming a shellac coating includes dissolving shellac in an aqueous-based ammonia solution. However, subsequent volatilization of ammonia from the solution to form a shellac coating, generates toxic vapors. Further, the viscosity of aqueous-based ammonia solutions with shellac increases significantly over a period of about three to four months. These solutions generally solidify after about six months, thereby significantly limiting their shelf-life. Also, coatings which have been formed from such solutions typically include amine-containing shellac salts and are generally not resistant to water, usually dissolving or forming opaque/white blemishes when the coatings are exposed to water. Further, such coatings have an adverse taste and are susceptible to microbial contamination.
In addition, current confectionery practice requires that a priming layer of gum arabic, or an equivalent material, be applied before coating with shellac. Similarly, the alkaline, water-based shellac solutions cannot be applied directly to chocolate to produce a continuous, acceptable film.
Therefore, a need exists for an improved coating and an improved method of coating articles, such as foods and medications, which overcome the above-mentioned problems.